I. Field of the Invention
The present invention pertains generally to the field of communications, and more specifically to constructing gain tables for mobile stations in a wireless communication network.
II. Background
The field of wireless communications has many applications including, e.g., cordless telephones, paging, wireless local loops, personal digital assistants (PDAs), Internet telephony, and satellite communication systems. A particularly important application is cellular telephone systems for mobile subscribers. (As used herein, the term xe2x80x9ccellularxe2x80x9d systems encompasses both cellular and personal communications services (PCS) frequencies.) Various over-the-air interfaces have been developed for such cellular telephone systems including, e.g., frequency division multiple access (FDMA), time division multiple access (TDMA), and code division multiple access (CDMA). In connection therewith, various domestic and international standards have been established including, e.g., Advanced Mobile Phone Service (AMPS), Global System for Mobile (GSM), and Interim Standard 95 (IS-95). In particular, IS-95 and its derivatives, IS-95A, IS-95B, ANSI J-STD-008, proposed high-data-rate systems for data, etc. (often referred to collectively herein as IS-95), are promulgated by the Telecommunication Industry Association (TIA) and other well known standards bodies.
Cellular telephone systems configured in accordance with the use of the IS-95 standard employ CDMA signal processing techniques to provide highly efficient and robust cellular telephone service. Exemplary cellular telephone systems configured substantially in accordance with the use of the IS-95 standard are described in U.S. Pat. Nos. 5,103,459 and 4,901,307, which are assigned to the assignee of the present invention and fully incorporated herein by reference. In CDMA systems, over-the-air power control is a vital issue. An exemplary method of power control in a CDMA system is described in U.S. Pat. No. 5,056,109, which is assigned to the assignee of the present invention and fully incorporated herein by reference.
A primary benefit of using a CDMA over-the-air interface is that communications are conducted over the same radio frequency (RF) band. For example, each mobile subscriber unit (e.g., a cellular telephone, personal digital assistant (PDA), laptop connected to a cellular telephone, hands-free car kit, etc.) in a given cellular telephone system can communicate with the same base station by transmitting a reverse-link signal over the same 1.25 MHz of RF spectrum. Similarly, each base station in such a system can communicate with mobile units by transmitting a forward-link signal over another 1.25 MHz of RF spectrum. Transmitting signals over the same RF spectrum provides various benefits including, e.g., an increase in the frequency reuse of a cellular telephone system and the ability to conduct soft handoff between two or more base stations. Increased frequency reuse allows a greater number of calls to be conducted over a given amount of spectrum. Soft handoff is a robust method of transitioning a mobile unit from the coverage area of two or more base stations that involves simultaneously interfacing with two base stations. In contrast, hard handoff involves terminating the interface with a first base station before establishing the interface with a second base station. An exemplary method of performing soft handoff is described in U.S. Pat. No. 5,267,261, which is assigned to the assignee of the present invention and fully incorporated herein by reference.
In conventional cellular telephone systems, a public switched telephone network (PSTN) (typically a telephone company) and a mobile switching center (MSC) communicate with one or more base station controllers (BSCs) over standardized E1 and/or T1 telephone lines (hereinafter referred to as E1/T1 lines). The BSCs communicate with base station transceiver subsystems (BTSs) (also referred to as either base stations or cell sites), and with each other, over a backhaul comprising E1/T1 lines. The BTSs communicate with mobile units via RF signals sent over the air.
To provide increased capacity, the International Telecommunications Union recently requested the submission of proposed methods for providing high-rate data and high-quality speech services over wireless communication channels. The submissions describe so-called xe2x80x9cthird generation,xe2x80x9d or xe2x80x9c3G,xe2x80x9d systems. An exemplary proposal, the cdma2000 ITU-R Radio Transmission Technology (RTT) Candidate Submission (referred to herein as cdma2000), was issued by the TIA. The standard for cdma2000 is given in draft versions of IS-2000 and has been approved by the TIA. The cdma2000 proposal is compatible with IS-95 systems in many ways.
One significant way in which the proposed cdma2000 system differs from an IS-95 system is that reverse-link traffic channels (i.e., the voice- or data-carrying signal from the mobile station to the base station) are demodulated coherently according to the cdma2000 proposal. Hence, a table of gains and pilot channel power levels for mobile stations is stored in each base station to enable the base station to specify traffic channel power levels for mobile stations with which the base station is communicating. The table contains different gains and pilot levels for each combination of data rate in bits per second (bps), target frame error rate (FER) (typically either one or five percent), type of forward error correction coding used by the mobile station (either convolutional coding or turbo coding), and frame length (twenty milliseconds (ms) for fundamental traffic channels and five, twenty, forty, or eighty ms for supplemental channels). The gains are expressed in decibels as ratios of the traffic channel level to the pilot channel level, multiplied by eight. An exemplary gain table is illustrated in 3rd Generation Partnership Project 2 xe2x80x9c3GPP2xe2x80x9d, xe2x80x9cPhysical Layer Standard for cdma2000 Spread Spectrum Systems,xe2x80x9d 3GPP2 Document No. C.P0002-A, TIA PN4694, to be published as TIA/EIA/IS-2000-2-A, (Draft, edit version 30), Table 2.1.2.3.3.2-1 (Nov. 19, 1999).
For a given target FER, the traffic to pilot ratio (gain) varies with the velocity of the mobile station. In the proposed cdma2000 submission, the traffic to pilot ratio for a pilot power level required for a given FER is calculated for three different possible mobile station velocities (high (e.g., 120 km/hr.), low (e.g., thirty km/hr.), and static (e.g., additive white Gaussian noise (AWGN))) and averaged. The resultant average value is stored in the gain table. The cdma2000 reverse link (mobile-station-to-base-station communication) thus uses predefined traffic to pilot ratios. In many cases, the optimal value for the best case may be two to three decibels (dB) lower than the value chosen, and if transmission were performed at the optimal level, it would substantially alter the power requirements for the mobile station and provide capacity gains. However, to transmit at that level, it would be necessary to estimate the velocity of the mobile station. It would be advantageous, therefore, to provide more accurate gains based on estimated velocities of the mobile station. Thus, there is a need for a method of modifying the gain tables for a mobile station in a velocity-dependent manner.
The present invention is directed to a method of modifying the gain tables for a mobile station in a velocity-dependent manner. Accordingly, in one aspect of the invention, a method of modifying a transmit power level of a mobile station in a wireless communication network is provided. The method advantageously includes the steps of estimating a velocity of the mobile station; and modifying a transmit power level of the mobile station based on the estimated velocity.
In another aspect of the invention, an infrastructure element in a wireless communication network is provided. The infrastructure element advantageously includes means for estimating a velocity of a mobile station in the wireless communication network; and means for modifying a transmit power level of the mobile station based on the estimated velocity.
In another aspect of the invention, an infrastructure element in a wireless communication network is provided. The infrastructure element advantageously includes a processor; and a processor-readable storage medium coupled to the processor and containing an instruction set executable by the processor to estimate a velocity of a mobile station in the wireless communication network and modify a transmit power level of the mobile station based on the estimated velocity.
In another aspect of the invention, an infrastructure element in a wireless communication network is provided. The infrastructure element advantageously includes a velocity estimation module configured to estimate a velocity of a mobile station in the wireless communication network; and a gain table modification module coupled to the velocity estimation module and configured to modify a gain table in the infrastructure element based on the estimated velocity, the gain table containing gains for use in modifying a transmit power level of the mobile station.